Cervical cancer is caused by human papillomavirus (HPV) infection [1]. Cervical cancer occurs in approximately 500,000 women every year, resulting in 250,000 deaths [2]. It has been assumed that most women have asymptomatic HPV infections at sometimes in their lives [3, 4].
HPV type 16 (HPV 16) is thought to be the most important type because it is more than 50% cause of cervical cancer cases and 90% cause of head and neck cancer [5]. L1 protein occupies 80-90% of HPV capsid and has property that self-assembles into virus-like particles (VLPs), the structure of which is similar to that of naturally occurring HPV virions [6]. Therefore, it has been regarded that immunization with HPV VLPs provokes neutralizing antibodies efficiently [7]. Currently, there are two types of VLP-based prophylactic vaccines commercially available. One is Gardasil™ (Merck) that is produced by Saccharomyces cerevisiae expression system, and the other Cervarix™ (GlaxoSmithKline) that is produced by insect cell expression system [8]. These vaccines contain HPV type 16 and 18 VLPs as antigens, adopt three doses immunization protocols and are thought to have effects for protecting 70% of cervical cancer theoretically because infections with HPV type 16 and 18 are responsible for cause of 70% of all cervical cancer cases [9]. The retail price of Gardasil® or Cervarix™ is approximately $120 per dose, and $360 for the full series [10]. The high retail prices of the current HPV vaccines have limited widespread use of them, and these vaccines are unaffordable in developing countries because of their high retail prices [11]. Therefore, the strategy for improving yield of the vaccine antigen, L1 protein, is a high priority in HPV vaccine field.
To secure the economic benefit during the biopharmaceutical manufacturing process, the improved productivity in bioreactor in the upstream process must lead to the improvement of the yield of final product [12]. However, little attention has been paid to this principle, and little study has been made to improve the yield rate by optimizing the culture condition in case of recombinant HPV L1 protein production. Moreover, study to compare the productivities of L1 proteins in varied culture conditions is practically difficult because the protocol for purifying the HPV L1 protein is time-consuming, labor-intensive and costly. Recently, we developed one-step chromatographic purification method for HPV L1 protein produced in Saccharomyces cerevisiae, which is suitable for large-scale production of HPV L1 protein and adoptable for various samples [13].
GAL10 promoter of Saccharomyces cerevisiae is the most powerful and frequently used promoter for heterogeneous protein production [14, 15]. Saccharomyces cerevisiae preferentially uses glucose if both glucose and galactose exist, and the GAL promoters are induced by galactose when the glucose system is repressed [16]. Therefore, the composition of carbon sources is the most important factor for producing recombinant protein under the GAL10 promoter system. [17-19]. Generally, the production of L1 protein in Saccharomyces cerevisiae requires 48-72 h of culture time and 2-4% carbon source that are composed of glucose and galactose [20-22]. However, there have been few reports on the result of improvement of expression yield of HPV L1 protein by changing the concentration and composition of the carbon source in the culture medium.
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